As an introduction to problems solved by the present invention, consider the conventional tape system used for storage and retrieval of binary data for a computer system. Such a tape system stores digital information as data transitions of magnetic flux on magnetic tape. These data transitions are separated by varying time periods, the ratio between the maximum and minimum time periods varying according to the data recording standard being used. For example, it is conventional to encode serial binary data as a series of time periods known as data bit cells by omitting a data transition from a cell for a binary zero and generating a data transition in a cell for each binary one. Further, the spacing of data transitions is controlled by encoding the binary data according to a standard of the type known as run length limited (RLL). One conventional RLL encoding, {1,7}, calls for a minimum separation of one binary zero between binary ones and a maximum separation of seven binary zeros between binary ones. Consequently, any RLL code designated {1,x} conveys binary bits without the occurrence of consecutive data bit cells each having a data transition. These constraints are easily applied to data to be recorded and greatly simplify detection of the recorded data transitions.
In reading data from a magnetic tape, the tape passes over a read head which provides a signal to circuitry of a read channel that detects a change of flux as a pulse. The amplitude and peak position of the pulse provided by the read channel varies substantially depending on the time period separating one data transition from another and depending on the pattern of data transitions. A read channel which cannot accommodate such variation is considered unreliable because the read channel fails to read recorded data without error. Two common failure modes of a read channel involve: (a) failing to detect a valid data transition when preceded by a comparatively long period of time devoid of data transitions; and (b) failing to detect one of two closely spaced data transitions.
When the time period between data transitions is large, the low frequency response of the read head can make detection of a valid pulse in a subsequent cell less reliable. According to schemes known as "write equalization", an additional pair of equalization transitions is introduced into cells corresponding to a zero data bit. These equalization transitions are placed so as not to be misread as a recorded one data bit. When the pair of equalization transitions is placed symmetrically within the time period of a cell, the write equalization scheme is considered symmetric as opposed to asymmetric.
When the time period between data transitions is short, pulse amplitude peaks are difficult to detect and the time between peaks is slightly increased. Conventional schemes known as "write precompensation" have been designed to perform peak separation by so called peak shift precompensation. Shift is accomplished using a multitap delay line or a shift register. By further separating data transitions during recording, peak shift is reduced and a lower error rate for consecutive cells has been achieved.
In general, a signal having two closely spaced transitions is said to suffer from intersymbol interference (ISI). Because each transition can be modeled by a Lorentzian pulse having a peak amplitude and tapering off-peak amplitudes, ISI can be understood from the superposition of such a pulse for each transition. After superposition, the peaks are separated further in time than without being combined into one signal. The closer the peaks are prior to superposition, the greater the peak shift; which is to say the greater the effect of ISI. Many known techniques have used to avoid ISI, including writing the closely spaced transitions further apart.
Markets served by data recording equipment have for years demanded and will continue to demand further increase in data bit packing density and increased read channel reliability. Further separation of data transitions works against increasing data bit packing density. Without the present invention, considerable expense is expected to be unnecessarily incurred for sophisticated head/media technology and complex read channel circuitry.
In view of the problems described above, the need remains for systems and methods of the type that provide a signal conveying data by the presence or absence of data transitions in addition to equalization transitions and that exhibit higher read channel reliability, higher data bit packing density, and lower cost than known alternative systems.